The present invention is an application of seismic techniques to medical imaging. A description of seismic art as currently practiced in geophysical oil and gas exploration will be followed by a discussion of weaknesses in current medical imaging technology. Certain geophysical and other terms are explained as follows:
1. Object is the word used to define the body being imaged in the lattice structure. It can apply to a human patient, an animal, a plant, or any specifically defined object which can be brought into the range of the lattice's field.
2. Acoustic Impedance Contrast or Reflection Coefficient is a way to calculate the amount of reflected sound energy and is a function of the contrast in density and wave propagation velocity.
3. Normal Move-Out is a velocity determination from and correction for changes in receiver offset from the source.
4. Gathers are groups of the traces from individual receivers.
5. A Brute Stack is a preliminary image of summed traces where assumptions, rather than interpretation of actual velocities, are used.
6. Isochrons form a map, measured in time, from the difference between two time surfaces, where one is subtracted from the other.
7. Isopachs are similar to Isochrons, but converted are to a distance difference from the time difference by using appropriate velocities.
8. Velocity Analyses are a method to display and interpret root-mean-squared velocities from interpretation and autocorrelation functions used on the gathers.
9. Constant Velocity Stacks are a series of images of the same plane where one velocity function has been used for the entire plane in each panel and progressively higher constant velocities in subsequent panels.
10. FK Filters are a mathematical method of noise reduction.
11. Noise is unwanted response and may be, but is not limited to, water disturbances, multiple reflections, diffraction curves, out-of-the-plane reflections, instrument malfunctions, random noise, and processing problems.
12. Stacking is a method of summation after gathers have been corrected for the horizontal displacement by normal move-out.
13. Migration is a mathematical means to move reflections to their proper place in three-dimensional space.
14. Cartesian Coordinate System is a referencing technique which identifies three orthogonal (each at right angles) axes, usually designated as x, y, and z. The center, or origin, of such a coordinate system is where each of the axes has a zero value (0,0,0).
Sound energy propagates as a three-dimensional wave, with primary, (or p-) waves, being manifested as successive compaction and rarefaction. If the energy travels through an anisotropic material, one with different p-wave velocities in different directions, the wave will not be spherical. When a sonic wave encounters a boundary between different materials, i.e. an abrupt change in density and wave propagation velocity, part of the energy is refracted through the boundary and part is reflected back. The greater the contrast between the materials at the boundary, the greater is the acoustic impedance, contrast or reflection coefficient the greater the percentage of sonic energy reflected back and the lesser the percentage of the energy refracted onward. At any acoustic boundary, the angle of incidence equals the angle of reflection. This is commonly represented by rays drawn perpendicular to the tangent of the wave.
The most significant advance in seismic art has been common depth point imaging recorded directly in digital form. Common depth point imaging is a data manipulation technique which is a summation of many signals from different receivers, correlating coherent wavelet responses of multiple source/receiver positions to a common reflection point. This summation has the effect of
1. enhancing the strength of the boundary reflection, PA1 2. reducing random noise through destructive interference, and PA1 3. interpreting root-mean-squared velocities. PA1 1. A significant number of patients do not meet eligibility requirements because of particular man-made objects within their bodies; PA1 2. An MRI of just one part of the body (e.g. head or heart) is slow (about 45 minutes). Patients with some medical problems may have difficulty remaining prone and still for that period; PA1 3. The equipment is very expensive, large, and not easily or widely available in many areas; PA1 4. Many people find an MRI to be intimidating, uncomfortable, and unpleasant, PA1 5. Effects of long-term (especially repeated) exposure to powerful magnetic fields are unknown. PA1 1. Images have a poor signal to noise ratio, with fuzzy reflections and grainy background; PA1 2. There is no systemic correction of velocity variations within observed tissue structures, leading to poor image resolution and ambiguities in interpretation; PA1 3. Aside from minor use of amplitude response and Doppler frequency changes, current ultrasound imaging systems have no ability to process for and make use of wavelet attribute analyses for determination of body/organ characteristics, PA1 4. There is a strong dependence on operator skill; PA1 5. Often, the scanning procedure for a patient is a lengthy one; PA1 6. Medical ultrasound lacks a rigorous time-to-depth conversion using known propagation velocities for the body as a whole; PA1 7. Only a small portion of the body can be imaged at once.
Commonly, multiple seismic sources emit sound energy simultaneously. These arrays of sources can be tuned, by a combination of phase, frequency, and amplitude of the emitted sonic pulse, both theoretically and empirically, for maximum effective penetration and imaging of the target.
The received signals from individual sonic receivers, set in a varying array (for example, multiple and multi-mile, flexible cables of towed hydrophones), are grouped into gathers so that normal move-out and spatial corrections can be applied. This allows interpretation of a signal to correct coherent reflections from increasingly horizontally offset receivers. In this manner, the reception time of the reflected energy signals from the same point, imaged from multiple angles and distances, becomes very close to the same for all angles and offset distances for that common depth point. Therefore, when these individually, digitally recorded, corrected wavelet traces are added, or stacked, together, boundary reflections are enhanced by constructive interference while random noise undergoes destructive interference. Thus, the signal to noise ratio is significantly, even dramatically, improved for substantially better imaging.
Analyses of multi-directional velocity data, which includes changes in reflection amplitude versus increasing offset (AVO) can, in particular circumstances, provide direct information on material composition and even fluid or gas content. Successful distinctions have been made on the gas or water content of microscopic pores between sand grains because of different horizontal and vertical wave velocities.
The problems associated with data interpretation include the unknown properties of the material through which the energy is propagating and its therefore unknown propagation velocity. Common industry practice is to interpret wave propagation velocity, then to vary the velocities in the computer while simultaneously observing the effects upon the gathers and the stacked traces. If wave velocities were known beforehand to a close approximation, much of this could be automated for a quick brute stack. Velocity analyses are also used to reduce or eliminate multiple reflections and diffractions. When correct diffraction velocities are found, point sources, like faults and fractures, can be determined.
A variety of mathematical algorithms are used to migrate reflections to their proper place in three-dimensional space.
Resulting, processed seismic data loaded onto a computer, are configured to appear as closely and regularly spaced vertical planes in two directions, with time-horizontal planes, making a three-dimensional volume. Navigation or surveying carefully establishes the position of every receiver within an established coordinate system, and hence, the position of associated, underlying reflection points. An interpreter will usually map the shape of surfaces. From any arbitrary or interpreted surface, many parallel slices can be generated for a quick look at wavelet attributes above and below the selected surfaces. Wave attributes (like amplitude, phase, coherence, frequency, multi-directional interval velocities, and calculated values like Poisson's ratio) can be automatically added over specified windows along the surface, isochrons and isopachs automatically constructed between selected surfaces, volumes automatically calculated, various mathematical functions applied to any set of x,y,z data points (referenced to the origin) to enhance discontinuities or amplitude anomalies, or to edit out known extraneous or erroneous data. Any set of x,y,z data points can be used in a mathematical function, or contoured and overlaid, with any other set of x,y,z points, not necessarily of seismic origin. Displays can be shaped for depth and colored for an attribute like amplitude. In a three-dimensional volume display, selected phases can be repeatedly faded into translucency or transparency in real time. Displays can be in the variable intensity format commonly used in most current medical imaging or in the wiggle-trace/variable area format sometimes used in geophysical displays.
The current art of medical imaging cannot provide a detailed, computer generated, three-dimensional image of the whole body of a particular patient. The current art of medical imaging of any type lacks a useable, repeatable x,y,z coordinate system for the body as a whole. The current art lacks the capability to manipulate signal response/wavelet attributes in a rigorous and substantial manner.
Magnetic resonance imaging (MRI), as currently available, has the capability to provide detailed, three-dimensional images, but only of selected portions of the anatomy. There are several other limitations to MRI. These include:
Ultrasound is an alternative medical imaging technology to MRI. However, the present state-of-the-art for ultrasound also has limitations, giving rise to a corresponding need for improvement. These include:
Radioactive imaging techniques are potentially harmful if used too often or at too strong an intensity